EP0923722A4 - Electrochemical cell - Google Patents

Abstract

An electrochemical sensor is described for determination of glucose in a liquid blood sample. This comprises a first sheet (14) having a palladium sputter coating, as well as a second sheet (15) having a palladium sputter coating and a third sheet of electrically resistive material having an aperture (11) extending through the third sheet and defining a side wall of the cell. The space between the first sheet and the second sheet comprises a capillary space. A ferricyanide mediator is supported on one of the coatings. Access to the cell is provided at the side edge of the cell where notches (16) extending through the first sheet, second sheet, and third sheet intersect cell (11). A liquid blood sample in contact with a notch is spontaneously drawn by capillary action into the electrochemical cell. <IMAGE>

Description

TITLE: ELECTROCHEMICAL CELL

FIELD OF THE INVENTION

This invention relates to an electrochemical cell for determining the

concentration of an analyte in a carrier. BACKGROUND ART

The invention herein described is an improvement in or modification of the

invention described in our co-pending application PCT/AU96/00365, the contents of which are incorporated herein by reference.

The invention will herein be described with particular reference to a biosensor

adapted to measure the concentration of glucose in blood, but it will be understood not to be limited to that particular use and is applicable to other analytic determinations.

It is known to measure the concentration of a component to be analysed in an aqueous liquid sample by placing the sample into a reaction zone in an electrochemical

cell comprising two electrodes having an impedance which renders them suitable for amperometric measurement. The component to be analysed is allowed to react directly or indirectly with a redox reagent whereby to form an oxidisable (or reducible) substance in an amount corresponding to the concentration ofthe component to be analysed. The quantity ofthe oxidisable (or reducible) substance present is then estimated

electrochemically. Generally this method requires sufficient separation ofthe electrodes

so that electrolysis products at one electrode cannot reach the other electrode and

interfere with the processes at the other electrode during the period of measurement.

In our co-pending application we described a novel method for determining the

concentration of the reduced (or oxidised) form of a redox species in an electrochemical

cell of the kind comprising a working electrode and a counter (or counter/reference)

electrode spaced from the working electrode by a predetermined distance. The method involves applying an electric potential difference between the electrodes and selecting the potential of the working electrode such that the rate of electro-oxidation ofthe reduced form of the species (or of electro-reduction of the oxidised form) is diffusion controlled. The spacing between the working electrode and the counter electrode is selected so that reaction products from the counter electrode arrive at the working

electrode. By determining the current as a function of time after application of the potential and prior to achievement of a steady state current and then estimating the magnitude of the steady state current, the method previously described allows the diffusion coefficient and/or the concentration of the reduced (or oxidised) form of the

species to be estimated. Our co-pending application exemplifies this method with reference to use of a

"thin layer electrochemical cell" employing a GOD/Ferrocyanide system. As herein

used, the term "thin layer electrochemical cell" refers to a cell having closely spaced electrodes such that reaction product from the counter electrode arrives at the working electrode. In practice, the separation of electrodes in such a cell for measuring glucose

in blood will be less than 500 microns, and preferably less than 200 microns.

The chemistry used in the exemplified electrochemical cell is as follows:

glucose + GOD — > gluconic acid + GOD* reaction 1

GOD* + 2ferricyanide → GOD + 2ferrocyanide reaction 2

where GOD is the enzyme glucose oxidase, and GOD* is the 'activated' enzyme.

Ferricyanide ([Fe(CN)₆] ) is the 'mediator' which returns the GOD* to its catalytic

state. GOD, an enzyme catalyst, is not consumed duπng the reaction so long as excess

mediator is present. Ferrocyanide ([Fe(CN)₆]^") is the product of the total reaction.

Ideally there is initially no ferrocyanide. although in practice there is often a small

quantity. After reaction is complete the concentration of ferrocyanide (measured

electrochemically) indicates the initial concentration of glucose. The total reaction is the

sum of reactions 1 and 2:

GOD glucose + 2ferricyanidc — gluconic acid + 2ferrocyanide reaction 3

"Glucose" refers specifically to β-D-glucose.

The prior art suffers from a number of disadvantages. Firstly, sample size

required is greater than desirable. It would be generally preferable to be able to make

measurements on samples of reduced volume since this in turn enables use of less

invasive methods to obtain samples.

Secondly, it would be generally desirable to improve the accuracy of

measurement and to eliminate or reduce variations due. for example, to cell asymmetry

or other factors introduced during mass production of microcells. Likewise, it would be

desirable to reduce electrode "edge" effects. Thirdly, since the cells are disposable after use, it is desirable that they be

capable of mass production at relatively low cost.

DISCLOSURE OF THE INVENTION

According to one aspect the invention consists in a method of manufacture of

an electrochemical cell comprising the steps of:

forming an aperture extending through a sheet of electrically resistive material.

said aperture defining a side wall of the cell;

mounting a first thin electrode layer to one side of the sheet and extending over

the aperture whereby to define a cell first end wall;

mounting a second thin electrode iayer to the other side of the sheet and

extending over the aperture whereby to define a cell second end wall in substantial

overlying registration with the first electrode; and

providing means for admission of a liquid into the cell defined between the side

wall and said end walls.

The first and second electrode layers may be conductors or semi-conductors and

may be the same or different. Noble metal electrode layers are preferred.

In preferred embodiments of the invention the aperture is of circular cross-

section whereby the side wall is cylindrical and the first and second electrodes cover the

aperture.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention will now be particularly described by way of example only with

reference to the accompanying schematic drawings wherein:

Figure 1 shows the product of manufacturing step 2 in plan.

Figure 2 shows the product of Figure 1 in side elevation. Figure 3 shows the product of Figure 1 in end elevation.

Figure 4 shows the product of manufacturing step 3 in plan.

Figure 5 shows the product of Figure 4 in cross-section on line 5-5 of Figure 4.

Figure 6 shows the product of manufacturing step 5 in plan.

Figure 7 shows the product of Figure 6 in side elevation.

Figure 8 shows the product of Figure 6 in end elevation.

Figure 9 shows the product of manufacturing step 7 in plan.

Figure 10 is a cross-section of Figure 9 on line 10-10.

Figure 1 1 shows the product of Figure 9 in end elevation.

Figure 12 shows a cell according to the invention in plan.

Figure 13 shows the call of Figure 12 in side elevation.

Figure 14 shows the cell of Figure 12 in end elevation.

Figure 15 shows a scrap portion of a second embodiment ofthe invention in enlarged section. The construction of a thin layer electrochemical cell will now be described by

way of example of the improved method of manufacture.

Step 1 : A sheet 1 of Melinex (a chemically inert, and electrically resistive Polyethylene Terephthalate ["PET"]) approximately 13cm x 30cm and 100 micron thick was laid flat on a sheet of release paper 2 and coated using a Number 2 MYAR bar to a

thickness of 12 microns wet (approximately 2-5 microns dry) with a water-based heat activated adhesive 3 (ICI Novacoat system using catalyst:adhesive). The water was then evaporated by means of a hot air dryer leaving a contact adhesive surface. The sheet was then turned over on a release paper and the reverse side was similarly coated with the

same adhesive 4, dried, and a protective release paper 5 applied to the exposed adhesive surface. The edges were trimmed to obtain a sheet uniformly coated on both sides with tacky contact adhesive protected by release paper.

Step 2: The sheet with protective release papers was cut into strips 7. each about 18mm x 210mm (Figures 1-3).

Step 3: A strip 7 of adhesive-coated PET from step 2 with release paper 2, 5 on

respective sides, was placed in a die assembly (not shown) and clamped. The die

assembly was adapted to punch the strip with a locating hole 10 at each end and with for example 37 circular holes 1 1 each of 3.4mm diameter at 5mm centres equi-spaced along a line between locating holes 10. The area of each hole 1 1 is approximately 9 square mm.

Step 4: A sheet 12 of Mylar^® PET approximately 21cm square and 135 microns thick was placed in a sputter coating chamber for palladium coating 13. The

sputter coating took place under a vacuum of between 4 and 6 millibars and in an atmosphere of argon gas. Palladium was coated on the PET to a thickness of 100-1000 angstroms. There is thus formed a sheet 14 having a palladium sputter coating 13.

Step 5: The palladium coated PET sheet 14 from Step 4 was then cut into strips

14 and 15 and a die was used to punch two location holes 16 in each strip, at one end

(Figures 6, 7 and 8). Strips 14 and 15 differ only in dimension strips 14 being 25mm x

210mm and strips 15 being 23mm x 210mm. Step 6: A spacer strip 7 prepared as in step 3 was then placed in a jig (not

shown) having two locating pins (one corresponding to each locating hole 10 of strip 7)

and the upper release paper 2 was removed. A strip 14 of palladium coated PET prepared as in step 5 was then laid over the adhesive layer, palladium surface downwards, using the jig pins to align the locating holes 16 with the underlying PET strip 7. This combination was then passed through a laminator comprising a set of pinch

rollers, one of which was adapted to heat the side bearing a palladium coated PET strip

14. The roller on the opposite side of the strip 7 was cooled. By this means, only the

adhesive between the palladium of strip 14 and PET strip 7 was activated.

Step 7: PET strip 7 was then turned over and located in the jig with the release

coating uppermost. The release coating was peeled off and second palladium coated

strip 15 was placed palladium side down on the exposed adhesive surface using the

locating pins to align the strips, this assembly was now passed again through the

laminator of step 6, this time with the hot roll adjacent the palladium coated Mylar*

added in step 7 so as to activate the intervening adhesive (Figures 9, 10 and 1 1).

Step 8: The assembly from step 7 was returned to the die assembly and notches

17 punched in locations so as to extend between the circular holes 1 1 previously

punched in the Melinex" PF.T and the strip edge 17. Notches 16 extend so as to

intercept the circumference of each circular cell. The strip was then guillotined to give

37 individual "sensor strips^", each strip being about 5mm wide and each having one thin

layer cavity cell (Figures 12. 13 and 14).

There is thus produced a cell as shown in Figures 12. 13 or 14. The cell

comprises a first electrode consisting of PET layer 12. a palladium layer 13, an adhesive

layer 3. a PET sheet 1 . a second adhesive layer 4. a second electrode comprising

palladium layer 13. and a PET layer 12. Sheet 1 defines a cylindrical cell 1 1 having a

thickness in the cell axial direction corresponding to the thickness of the Melinex PET

sheet layer 1 together with the thickness of adhesive layers 3 and 4. The cell has circular

palladium end walls. Access to the cell is provided at the side edge of the cell where

notches 16 intersect cell 1 1. In preferred embodiments of the invention, a sample to be analysed is

introduced to the cell by capillary action. The sample is placed on contact with notch 16

and is spontaneously drawn by capillary action into the cell, displaced air from the cell

venting from the opposite notch 16. A surfactant may be included in the capillary space

to assist in drawing in the sample.

The sensors are provided with connection means for example edge connectors

whereby the sensors may be placed into a measuring circuit. In a preferred embodiment

this is achieved by making spacer 1 shorter than palladium supporting sheets 14. 15 and

by making one sheet 15 of shorter length than the other 14. This forms a socket region

20 having contact areas 21. 22 electrically connected with the working and counter

electrodes respectively. A simple tongue plug having corresponding engaging conduct

surfaces can then be used for electrical connection. Connectors of other form may be

devised.

Chemicals for use in the cell may be supported on the cell electrodes or walls,

may be supported on an independent support contained within the cell or may be self-

supporting.

In one embodiment, chemicals for use in the cell are printed onto the palladium

surface of the electrode immediately after step 1 at which stage the freshly-deposited

palladium is more hydrophilic. For example, a solution containing 0.2 molar potassium

ferricyanide and 1 % by weight of glucose oxidase dehydrogenase may be printed on to

the palladium surface. Desirably, the chemicals are printed only in the areas which will

form a wall of the cell and for preference the chemicals are printed on the surface by

means of an ink jet printer. In this manner, the deposition of chemicals may be precisely

controlled. If desired, chemicals which are desirably separated until required for use may be printed respectively on the first and second electrodes. For example, a

GOD/ferrocyanide composition can be printed on one electrode and a buffer on the other. Although it is highly preferred to apply the chemicals to the electrodes prior to assembly into a cell, chemicals may also be introduced into the cell as a solution after

step 6 or step 8 by pipette in the traditional manner and the solvent subsequently is

removed by evaporation or drying. Chemicals need not be printed on the cell wall or the electrodes and may instead be impregnated into a gauze, membrane, non-woven fabric or the like contained within, or filling, the cavity (eg inserted in cell 1 1 prior to steps 6 or 7). In another embodiment the chemicals are formed into a porous mass which may be

introduced into the cell as a pellet or granules. Alternatively, the chemicals may be introduced as a gel.

In a second embodiment of the invention a laminate 21 is first made from a strip

14 as obtained in step 5 adhesively sandwiched between two strips 7 as obtained from step 3. Laminate 20 is substituted for sheet 1 in step 5 and assembled with electrodes as

in steps 6 and 7.

There is thus obtained a cell as shown in Figure 15 which differs from that of

Figures 9 to 1 1 in that the cell has an annular electrode disposed between the first and second electrode. This electrode can for example be used as a reference electrode. It will be understood that in mass production of the cell, the parts may be assembled as a laminate on a continuous line. For example, a continuous sheet 1 of PET

could be first punched and then adhesive could be applied continuously by printing on the remaining sheet. Electrodes (pre-printed with chemical solution and dried) could be fed directly as a laminate onto the adhesive coated side. Adhesive could then be applied to the other side of the punched core sheet and then the electrode could be fed as a laminate onto the second side.

The adhesive could be applied as a hot meit interleaving film. Alternatively, the core sheet could first be adhesive coated and then punched.

By drying chemicals on each electrode prior to the gluing step the electrode surface is protected from contamination.

Although the cell has been described with reference to Mylar" and Melinex^® PET, other chemically inert and electrically resistive materials may be utilised and other dimensions chosen. The materials used for spacer sheet 1 and for supporting the reference and counter electrodes may be the same or may differ one from the other.

Although the invention has been described with reference to palladium electrodes, other metals such as platinum, silver, gold, copper or the like may be used and silver may be

reacted with a chloride to form a silver/silver chloride electrode or with other halides.

The electrodes need not be of the same metal. Although the use of heat activated adhesives has been described, the parts may

be assembled by use of hot melt adhesives. fusible laminates and other methods.

The dimensions of the sensor may readily be varied according to requirements.

While it is greatly preferred that the electrodes cover the cell end openings, in other embodiments (not illustrated) the electrodes do not entirely cover the cell end openings. In that case it is desirable that the electrodes be in substantial overlying

registration.

Preferred forms ofthe invention in which the electrodes cover the apertures of cell 1 1 have the advantages that the electrode area is precisely defmed simply by punching hole 1 1. Furthermore the electrodes so provided are parallel, overlying, of

substantially the same area, and are substantially or entirely devoid of "edge" effects.

Although in the embodiments described each sensor has one cell cavity⁷, sensors

may be provided with two or more cavities. For example, a second cavity may be

provided with a predetermined quantity ofthe analyte and may function as a reference

cell.

As will be apparent to those skilled in the art from the teaching herein contained, a feature of one embodiment herein described may be combined with features

of other embodiments herein described or with other embodiments described in our co-

pending application. Although the sensor has been described with reference to palladium electrodes and a GOD/ ferrocyanide chemistry, it will be apparent to those skilled in the art that other chemistries, and other materials of construction may be

employed without departing from the principles herein taught.

Claims

CLAIMS:-

1. A method of manufacture of a thin iayer electrochemical cell (as herein defined)

comprising the steps of:

forming an aperture extending through a sheet of electrically resistive material, said aperture defining a side wall ofthe cell,

mounting a first thin electrode layer to one side of the sheet and extending over

the aperture whereby to define a cell first end wall, mounting a second thin electrode layer to the other side of the sheet and

extending over the aperture whereby to define a second cell end wall in substantial overlying registration with the first electrode, and providing means for admission of a liquid into the cell defined between the side wall and

said end walls.

2. A method according to claim 1 wherein at least one of the electrode layers is a

noble metal. 3. A method according to claim 2 wherein at least one of the electrode layers is

palladium.

4. A method according to any one of claims 1 to 3 wherein each ofthe metal

electrode layers substantially covers the aperture.

5. A method according to any one of the preceding claims wherein the aperture is of circular cross-section, whereby the side- wall is cylindrical.

6. A method according to any one ofthe preceding claims wherein the means for admission of a liquid comprise a slot or passage communicating between the cell interior

and an edge ofthe sheet. 7 A method according to any one the preceding claims wherein the thin electrode

is a sputter coated metal deposit

8 A method according to any one ofthe preceding claims wherein the electrode

layers are adhered to the sheet

9 A method of manufacture of an electrochemical cell comprising the steps of

depositing a first sputter coating of a metal on a first suitable substrate, depositing a second sputter coating of the same or a different metal on a second suitable substrate, forming an aperture in a sheet of electrically resistive material, said aperture defining a side wall ol the cell and having openings on each face of the sheet adhering the first substrate to the sheet with the metal coating thereof adjacent one side of the sheet whereby to expose a portion of the first coating to one aperture

opening and to provide a first electrode adhering the second substrate to the sheet with the metal coating thereof adjacent the other side of the sheet whereby to expose a portion of the second coating to the other aperture opening facing the first and to provide a second electrode, and providing means for admission of a liquid into the cell defined between the side wall and said exposed portions of metal coating

10 A method according to claim 9 wherein the means for admission of a liquid comprise a passage communicating between the cell interior and an edge ofthe sheet

1 1 A method according to any one of claims 9 or 10 wherein the first sputter coating and the second sputter coating are adhered to the sheet by means of an adhesive

12 A method according to claim 11 wherein the adhesive is a heat activated

adhesive 13. A method according to any one of the preceding claims wherein one or more chemicals for use in the cell are printed onto one or both electrodes.

14. A method according to any one of the preceding claims wherein one or more chemicals for use in the cell are added in a solvent to the cell after one electrode has been

adhered to the sheet and before the other electrode has been adhered to the sheet and

wherein the solvent is subsequently evaporated to leave the chemicals in dry form in the cell.

15. A method according to any one ofthe preceding claims wherein one or more chemicals for use in the cell are contained in or on a support which is included within the cell.

16. A method according to any one of the preceding claims wherein one or more chemicals for use in the cell are contained within the cell in self-supporting form.

17. A method according to any one of the preceding claims wherein one or both electrodes comprise a sputter coated metal and one or more chemicals for use in the cell

are printed onto the metal surface immediately after the metal is sputter coated.

18. A method according to claim 17 wherein one or more chemicals for use in the

cell are printed onto the metal surface by means of an inkjet printer.

19. A method according to any one ofthe preceding claims wherein a plurality of first sputter coatings of metal are formed on a first substrate, a plurality of second sputter coatings of metal are deposited on a second substrate, a plurality of apertures are defined

in a sheet of electrically resistant material, each aperture defining a side wall of a cell,

laminating the first substrate to the sheet with the metal coatings thereof adjacent one side ofthe sheet and defining first end walls of respective cells defined by the sheet, and adhering the second substrate to the sheet with the metal coatings thereof adjacent the other side ofthe sheet and defining second cell end walls of respective cells defined in

the sheet, and subsequently dividing the assembly into a plurality of cells.

20. A method according to any one ofthe preceding claims, wherein one or more of the parts is provided with a formation adapted to interlock with a formation of another one of the parts or of an assembly jig.

21. A method according to any one of the preceding claims, wherein the sheet of electrically resistant material comprises a laminate having two layers, each of electrically and chemically resistant material. having a metal layer therebetween which in the final

product is useful as a reference electrode disposed inteπnediate the first metal and the

second metal.

22. An electrochemical cell comprising an electrically resistive substrate having a first thin layer of first metal on one face, a second electrically resistive substrate having a

second thin layer of metal sputter coated on the other face, said substrates being disposed

with the metal coating of one facing the metal coating of the other and being separated by a sheet pierced by an aperture, the wall of which aperture cooperates with said metal

coatings to define a cell wall, and an aperture whereby a solution may be introduced into the cell.

23. A method according to any one ofthe preceding claims, wherein the substrate is

a Polyethylene Terephthalate. 24. A method according to any one of the preceding claims, wherein the sheet is of

Polyethylene Terephthalate.

25. A method according to any one ofthe preceding claims, wherein at least one of

the electrode layers comprises palladium. 26. A method according to any one ofthe preceding claims, wherein one ofthe sputtered metal layers is of silver.

27. A method according to any one ofthe preceding claims when one ofthe layers

is a metal coating of silver and is halogenised to produce a silver halide surface.

28. A method according to any one of me preceding claims further comprising a second aperture piercing the sheet, and wherein the second aperture is covered on at least one side ofthe sheet by the same metal layer as covers the first aperture on that side.

29. A method according to anyone of claims 1 to 27 further comprising a second aperture wherein the second aperture is covered on at least one side by a metal layer which is electrically insulated from the metal layer covering the first aperture on that side.

30. A method according to claim 1, further comprising the step of providing means

for venting of gas during the admission of liquid into the cell.

31. A method according to claim 6, wherein the slot or passage is formed through the side wall and through the sheet of electrically resistive material.

32. A method according to any one of the preceding claims wherein the second thin electrode layer is mounted in opposing relationship a distance of less than 200 microns

from the first thin electrode layer.

33. A thin layer electrochemical cell comprising an electrically resistive sheet pierced by an aperture, a first electrode layer covering the aperture on one side ofthe sheet,

a second electrode layer covering the aperture on the other side ofthe sheet, and

a passage for admission into the aperture of a liquid. 34. A cell according to claim 33 wherein at least one electrode layer comprises a

conductor.

35. A cell according to claims 33 or 34 wherein at least one electrode layer

comprises a semi-conductor. 36. A cell according to any one of claims 33 to 35 wherein at least one electrode

layer comprises a noble metal.

37. A cell according to any one of claims 33 to 36 wherein at least one electrode

coating is palladium.

38. A cell according to any one of claims 33 or 37 wherein an electrode comprises a sputter coated metal.

39. A cell according to any one of claims 33 to 38 wherein the cell contains one or

more reagents.

40. A cell according to claim 33 wherein at least one reagent is provided on a

sputter coated metal electrode. 41. A cell according to claims 33 or 40 wherein one or more reagents are supported

or impregnated in a carrier within the cell.

42. A cell according to any one of claims 33 to 41 wherein one or more chemicals for use in the cell are contained within the cell in self-supporting form.

43. A cell according to any one of claims 33 to 42 wherein at least one of the electrodes is a sputter coated palladium.

44. A cell according to any one of claims 33 to 43 wherein the electrodes are of

substantially the same area.

45. A cell according to any one of claims 33 to 44 wherein the electrodes are in

substantial registration one with the other. 46. A cell according to claim 33 further comprising means for venting of gas during

admission of liquid into the cell.

47. A cell according to any one of claims 33 to 46 wherein the second electrode

layer is mounted in opposing relationship a distance of less than 200 microns from the

first electrode layer.